In recent years, optical filters that sufficiently transmit light in a visible wavelength region therethrough but shield light in a near infrared wavelength region are used for various applications.
For example, in imaging devices using a solid-state imaging element (e.g., CCD, CMOS, etc.), such as a digital still camera, a digital video camera, etc., or in display devices using a light receiving element, such as an automatic exposure meter, etc., in order to approximate the sensitivity of the solid-state imaging element or light receiving element to the luminosity factor of a human being, such an optical filter is disposed between an imaging lens and the solid-state imaging element or light receiving element. In addition, in PDP (plasma display panel), in order to prevent malfunction of a household electrical appliance remote controller operating at a near infrared ray from occurring, the optical filter is disposed on the front (viewing side) thereof.
Among them, as an optical filter for imaging device, glass filters in which CuO or the like is added to a fluorophosphate-based glass or a phosphate-based glass so as to selectively absorb light in a near infrared wavelength region are known. However, light absorption type glass filters involved such a problem that not only they are expensive, but also they are difficult to achieve slimming, so that they may be impossible to sufficiently respond to downsizing and slimming requirements for imaging devices of recent years.
Then, in order to solve the foregoing problem, there have been developed a reflection type interference filter in which, for example, a silicon oxide (SiO2) layer and a titanium oxide (TiO2) layer are stacked alternately on a substrate and which reflects light in a near infrared wavelength region due to the interference of light to shield the light; a film containing a dye that absorbs light in a near infrared wavelength region in a transparent resin; and the like (see, for example, Patent Document 1). In addition, there has also been developed an optical filter composed of a combination of the foregoing, in which a resin layer containing a dye that absorbs a near infrared ray and a layer that reflects a near infrared ray are stacked (see, for example, Patent Document 2). Furthermore, the resin layer containing a dye that absorbs a near infrared ray is described in, for example, Patent Document 3.
However, in these conventional optical filters for imaging device, the performance for shielding light of a wavelength in a near infrared region and the transmission properties in a wavelength band (630 to 700 nm) required for taking a picture of a dark portion more bright are not sufficient, and furthermore, the layer formation is restricted such that the functions of a solid-state imaging element are not hindered. Therefore, it is the present state that no optical filter having a sufficient near infrared ray cut filter function is obtainable.
On the other hand, a near infrared ray absorbing dye which exhibits a maximum absorption wavelength in the vicinity of from 700 to 750 nm and in which the inclination of an absorption curve of light having a wavelength of from 630 to 700 nm is steep, is used in a near infrared ray cut filter as a resin layer in which it is dispersed in a transparent resin, for example, a cycloolefin resin, in view of the matter that when used in combination with other shielding component or shielding member, good near infrared ray shielding properties are obtainable. However, such a near infrared ray absorbing dye involved such problems that its near infrared ray absorbing wavelength region is narrow, so that in many cases, even when combined with other shielding member, a wavelength region in which the absorption is not sufficient appears.